Foam separation

According to the collection procedure for the enriched gas-liquid and/or gas-solid interfaces, adsorptive bubble separation processes or techniques can also be divided into two large categories (a) foam separation, which involves the production of foam in the process, and (b) nonfoaming adsorptive bubble separation, which involves no production of foam. Foam separation can be further subdivide into foam fractionation and flotation. Nonfoaming adsorptive bubble separation, however, can be further subdivided into bubble fractionation, solvent sublation, and noirfoaming flotation. Lemlich (84) and Wang (1,75) provided the definitions of these technical terms in their books. 

Flotation is the term used to describe a process in which the species being separated from the bulk liquid media are insoluble particulates. Froth flotation is another one of the two foam separation processes. It also involves the production of foam in a heterogeneous aqueous system, and has a great deal of potential for the water and wastewater treatment. Froth flotation can be subdivided into at least seven categories (42,43,84), including precipitate flotation, ion flotation, molecular flotation, microflotafion, adsorption flotation, ore flotation, macroflotation, and adsorbing colloid flotation. They are described separately below. 

The term ion flotation is applied to the process in which there is an initially homogeneous solution that becomes heterogeneous after the addition of an oppositely charged surfactant due to the reaction between the surfactant and specific ion(s) thus, insoluble complexes are formed. These insoluble complexes will then attach themselves to the 

Molecular flotation is very similar to ion flotation, except that the surfactant forms an insoluble complex with a nonsurface-active molecule (i.e., not an ion) and the product is floated out. Because the formation of dipolar complexes molecular flotation is much weaker than that of ion-pair complexes in ion flotation, to date, almost all the work has involved flotation of ions using an oppositely charged surfactant (59). 

Adsorption flotation involves the removal of dissolved pollutants by activated carbon in a bubble reactor, and subsequent removal of activated carbon as well as other suspended particles by flotation technique (71). This process was found efficient for removing both dissolved organics and suspended solids from an industrial effluent (72), and for removing the emulsifled oil from water (73). The mechanism of removal has been proposed by Wang (72,73). 

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Foam separation Foam stability FoamulaR Focused ion beams Fodder radish Fog... 

Ion Flotation and Foam Separation. Ions and dissolved surfactant molecules can be removed from solutions by the agency of foam. In this case ions are sandwiched in foam films. The scientific basis of these processes is weU understood and successes of metal ion recovery from solutions including U, Pt, Au, as weU as different surfactants (detergents) have been reported in the Hterature. 

For small instaUafions, column foam separators are more suitable. Waste flows downward in the column whereas gas spargers, located at the bottom, give countercurrent flow. The foam generated is carried upward to a foam breaker and coUector. 

The second type is a stable dispersion, or foam. Separation can be extremely difficult in some cases. A pure two-component system of gas and liquid cannot produce dispersions of the second type. Stable foams can oe produced only when an additional substance is adsorbed at the liquid-surface interface. The substance adsorbed may be in true solution but with a chemical tendency to concentrate in the interface such as that of a surface-active agent, or it may be a finely divided sohd which concentrates in the interface because it is only poorly wetted by the liquid. Surfactants and proteins are examples of soluble materials, while dust particles and extraneous dirt including traces of nonmisci-ble liquids can be examples of poorly wetted materials. 

Among the methods of foam separation, foam fractionation usually implies the removal of dissolved (or sometimes colloidal) material. The overflowing foam, after collapse, is called thefoamate. The solid lines of Fig. 22-42 illustrate simple continuous foam fractionation. (Batch operation would be represented by omitting the feed and bottoms streams.)... 

Both types of coalescence can be important in the foam separations characterized by low gas flow rate, such as batchwise ion flotation producing a scum-bearing froth of comparatively long residence time. On the other hand, with the relatively higher gas flow rate of foam fractionation, the residence time may be too short for the first type to be important, and if the foam is sufficiently stable, even the second type of coalescence may be unimportant. 

Systems Separated Some of the various separations reported in the literature are listed in Rubin and Gaden, Foam Separation, in Schoen (ed.). New Chemical Engineering Separation Techniques, Interscience, New York, 1962, chap. 5 Lemlich, Ind. Eng. Chem., 60(10), 16 (1968) Pushkarev, Egorov, and Khrustalev, Clarification andDeactiva-tion of Waste Waters by Frothing Flotation, in Russian, Atomizdat, Moscow, 1969 Kuskin and Golman, Flotation of Ions and Molecules, in Russian, Nedra, Moscow, 1971 Lemlich (ed.), Adsorptive Bubble... 

Separation Techniques, Academic, New York, 1972 Lemhch, Adsub-ble Methods, in li (ed.). Recent Developments in Separation Science, vol. 1, CRC Press, Cleveland, 1972, chap. 5 Grieves, Chem. Eng. J., 9, 93 (1975) Valdes-Krieg King, and Sephton, Sep. Purif Methods, 6,221 (1977) Clarke and Wilson, Foam Flotation, Marcel Deldcer, New York, 1983 and Wilson and Clarke, Bubble and Foam Separations in Waste Treatment, in Rousseau (ed.). Handbook of Separation Processes, Wiley, New York, 1987. 

Wallace Jr GT, Wilson DF (1969) Foam separations as a tool in chemical oceanography. Report no. 6958. US Naval Research Laboratory, p 1... 

The foam produced with Aqueous Film Forming Foam (AFFF) concentrate is dry-chemical compatible. Protein and fluoroprotein foam concentrates and AFFF concentrates are incompatible and should not be mixed although foams separately generated with these concentrates are compatible and can be applied to a fire in sequence or simultaneously. AFFF is available in various liquid concentrate percentages. 

This means if one, through foam bubbles, collected the foam continuously, then more and more surface-active substances will be removed. Such a method of bubble foam separation has been used to purify wastewater from surface-active substances. It is especially useful when very minute amounts of surface-active substances (dyes in the printing industry pollutants in wastewater). The method is economical to use and is free of any chemicals or filters. In fact, if the pollutant is very expensive or poisonous, then this method can have many advantages over the other methods. 

FIGURE 8.6 Bubble foam separation method for wastewater purification. 

Foams are dispersions of gas in a relatively small amount of liquid. When they are still on the surface of the liquid from which they were formed, they also are called froths. Bubbles range in size from about 50 pm to several mm. The data of Table 19.8 show densities of water/air foams to range from 0.8 to 24 g/L. Some dissolved or finely divided substances may concentrate on the bubble surfaces. Beer froth, for instance, has been found to contain 73% protein and 10% water. Surface active substances attach themselves to dissolved materials and accumulate in the bubbles whose formation they facilitate and stabilize. Foam separation is most effective for removal of small contents of dissolved impurities. In the treatment of waste waters for instance, impurities may be reduced from a... 

TABLE 19.8. Data of Foam Separation Experiments Made in a 1 in. Dia Column on a Waste Water Containing Radioactive Components and Utilizing Several Different Surfactants... 

Y. Okamoto and E.J. Chou, Foam separation processes, in Handbook of Separation Techniques for Chemical Engineers (P. A. Schweitzer, Ed.), McGraw-Hill, New York, 1979, pp. 2.183-2.197. 

The second type is a stable dispersion, or foam. Separation can be extremely difficult in some cases. A pure two-component system of gas and liquid cannot produce dispersions of the second type. Stable foams can be produced only when an additional substance is adsorbed... 

In contrast, most of the conventional foam separation techniques use large bubbles, requiring relatively high gas flow rates to generate sufficient interfacial area for adhesion of solid particles to bubbles. This causes turbulence at the foam/liquid boundary and, in order to prevent redispersion of floated particles, a rather tall foam column is required (ref. 36). 

A.J. Rubin, E.A. Cassell, O. Henderson, J.D. Johnson and J.C. Lamb, Microflotation new low gas flow rate foam separation technique for bacteria and algae, Biotech Bioengng. 8 (1966) 135-150. 

The Vessize program next proceeds to calculate the required horizontal vessel lengths for gas bubble or foam separation from the oil phase. Water separation from the oil phase is also calculated in the following discussion... 

Gas bubble in oil phase. Little research here has been accomplished, and very little has been published about gas bubble or foam separation from liquid. Herein I offer a good contribution to this technology, along with a plea for more field-proven data. As in the case for liquid droplet fall in the gas phase, I propose that the same equations, Eqs. (4.5), (4.6), and (4.7), be used in the oil media. This is done in these three equations, Eq. (4.7) deriving the gas bubble terminal velocity. We must, however, input a feasible and proven gas particle size Du, pm. Having accomplished several field-proven foam separation tests, the following Dm determination equation is offered. 

The process in which an emulsion or foam separates. Usually coalescence causes the separation of a macrophase, and eventually the formerly dispersed phase becomes a continuous phase, separate from the original continuous phase. 

A simple and largely applied method for foam formation is dispersion of gas through porous plates (filters) placed at the lower parts of foam generation apparatus [5-10], This method is employed in flotation, in gas adsorption and dust collection in set-up with turbulent gas emulsion, and in the equipment for foam separation. The dispersity of a foam thus obtained depends on filter pore size or capillary diameter, hydrophility of the material used in the dispersion device construction, physicochemical properties of the foaming solution (surface tension, viscosity, surfactant concentration, etc.) and conditions of the dispersion process. 

Foam films are usually used as a model in the study of various physicochemical processes, such as thinning, expansion and contraction of films, formation of black spots, film rupture, molecular interactions in films. Thus, it is possible to model not only the properties of a foam but also the processes undergoing in it. These studies allow to clarify the mechanism of these processes and to derive quantitative dependences for foams, O/W type emulsions and foamed emulsions, which in fact are closely related by properties to foams. Furthermore, a number of theoretical and practical problems of colloid chemistry, molecular physics, biophysics and biochemistry can also be solved. Several physico-technical parameters, such as pressure drop, volumetric flow rate (foam rotameter) and rate of gas diffusion through the film, are based on the measurement of some of the foam film parameters. For instance, Dewar [1] has used foam films in acoustic measurements. The study of the shape and tension of foam bubble films, in particular of bubbles floating at a liquid surface, provides information that is used in designing pneumatic constructions [2], Given bellow are the most important foam properties that determine their practical application. The processes of foam flotation of suspensions, ion flotation, foam accumulation and foam separation of soluble surfactants as well as the treatment of waste waters polluted by various substances (soluble and insoluble), are based on the difference in the compositions of the initial foaming solution and the liquid phase in the foam. Due ro this difference it is possible to accelerate some reactions (foam catalysis) and to shift the chemical equilibrium of some reactions in the foam. The low heat... 

It has been suggested the methods of separation involving adherence of particles of different dispersity on bubble surface as a result of adsorption or adhesion, to be named adsorptive-bubble (adsubble) methods [27,28]. The methods of surface separation are termed differently in the different publications in the case of surfactant extraction they are referred to as adsorption flotation, foam flotation, foam fractionation, foam separation or adsorptive accumulation in the case of ion extraction, they are called ion flotation, foam flotation of hydrophobic precipitates, etc. 

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